Differential gas sensing in-line monitoring system

ABSTRACT

An in-line detector system and method for real-time detection of impurity concentration in a flowing gas stream. In a specific aspect, the system may comprise an in-line monitoring system for determining the calibrated concentration of an impurity species in a flowing gas stream, in a low concentration range below a predetermined concentration value, and in a high concentration range above said predetermined concentration value. The system may utilize hygrometric sensors in the case of water as a critical impurity, or surface acoustical wave (SAW) devices coated with suitable impurity-affinity coatings. The method includes sensing the impurity species concentration in gas derived from the gas flow stream in a sequential and repetitive sensing operation. In a first sensing mode the gas is purified of impurity species prior to sensing thereof and in a second sensing mode the gas is unpurified. In the first sensing mode and second sensing mode, the cycle times are varied in accordance with the impurity species concentration. In the low concentration range the second sensing mode cycle time is longer than the first sensing mode cycle time and in the high concentration the first sensing mode cycle time is longer than the second sensing mode cycle time.

This application is a continuation-in-part of prior U.S. patentapplication Ser. No. 07/930,184 filed Aug. 17, 1992 in the name of GlennM. Tom, to issue Jul. 5, 1994 as U.S. Pat. No. 5,325,705, which in turnwas a continuation-in-part of U.S. patent application Ser. No.07/628,490, filed Dec. 14, 1990 in the name of Glenn M. Tom, and issuedAug. 18, 1992 as U.S. Pat. No. 5,138,869.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to means and method for detecting the calibratedconcentration of an impurity species in a flowing gas stream, in a lowconcentration range, and in a high concentration range, with rapidrecovery to normal operation after exposure to high concentrations ofimpurity.

2. Description of the Related Art

The rapid expansion of vapor-phase processing techniques, e.g., chemicalvapor deposition, in the semiconductor industry has been associated withthe deployment and use of manufacturing equipment which is totallyreliant on the delivery of ultra-high purity process gases at the pointof use in the semiconductor manufacturing facility. Currently, over 5billion dollars worth of such equipment is in use.

Despite the widespread commercial employment of such vapor-phaseprocessing equipment, little effort has been focused to date on thedevelopment of systems for monitoring purity of gas streams in theprocess system.

As a result of the absence of commercially suitable gas impuritymonitoring systems, there is a recurrence of circumstances where a largenumber of wafers have been processed in the vapor-phase depositionreactor before it is recognized that compositional changes in theprocess gas stream flowed to the reactor are leading to high rates ofrejection. Such high rates of rejection in turn significantly lower theefficiency and productivity of the semiconductor manufacturing plant,and generate substantial losses of potential product. The resultingoff-spec microcircuitry articles thus constitute scrap which must bereworked, if this is even feasible, or else discarded as waste.

Accordingly, there is a pressing need in the semiconductor manufacturingindustry to provide commercially viable systems for continuouslymeasuring gas purity at the point of use. Such purity measurements canbe used to alter process conditions that would otherwise lead toproduction problems, e.g., by diverting the impurity-containing gasstream to suitable treatment prior to its ultimate use in the depositionprocess.

In the context of general industrial processes, such as petroleumrefining, wastewater treatment, biopharmaceutical production, etc., avariety of impurity monitoring and detection systems have been developedto detect fluid phase impurities, using sampling of a side stream, orslip stream, of the main flow stream for impurity concentrationdetermination. The sampled side stream typically is flowed through themonitoring and detection apparatus and then discarded. In the field ofsemiconductor manufacture, such wastage is highly detrimental to theeconomics of the semiconductor production process when the gas stream,as is generally the case, contains costly reagent materials, e.g.,organometallic source reagents for metal deposition on a substrate.Further, many gas streams employed in semiconductor manufacturing arehighly hazardous in character, so that their waste presents significantdifficulties in handling, treatment, and disposal.

Considering the impurities which are present in gas streams involved insemiconductor manufacturing, it is to be noted that the growth of highquality thin film electronic and opto-electronic cells by chemical vapordeposition or other vapor-based techniques is inhibited by a variety oflow-level process impurities. These impurities affect both productsemiconductor defects and yield.

Specifically, at least two types of contamination are significant, viz.,particulate contamination and chemical contamination. Particulatecontamination has been successfully addressed by a variety of filtrationand collection methods and apparatus (see Malczewski, M. L., et al,"Measurement of Particulates in Filtered Process Gas Streams," SolidState Technology, 28, 151-157, April 1986). Chemical contamination hasnot received similar attention. As mentioned, the monitoring deviceswhich have been developed in other industries are ill-suited forapplication to semiconductor manufacturing operations.

In the semiconductor manufacturing operation, chemical impurities inreactive process gases can originate in the production of the source gasitself, as well as in its subsequent packaging, shipment, storage, andhandling. Although source gas manufacturers typically provide analysesof source gas materials delivered to the semiconductor manufacturingfacility, the purity of such gases may change. Such change may be due toleakage into or outgassing of the containers, e.g., gas cylinders,employed to package such gases. Alternatively, impurity contaminationmay result from improper gas container changes, leaks into downstreamprocessing equipment, or outgassing of such downstream equipment.

Accordingly, the only comprehensive solution for consistent delivery ofhigh purity gases for vapor processing operations in semiconductormanufacture is the development of commercially useful impurity detectionsystems for real-time measurement of critical impurity concentrations insemiconductor manufacturing process streams and the deployment ofreliable point-of-use purification systems for purifying gas streamswhich are determined to contain impurity species in excess of allowableconcentrations.

The presence of even small concentrations of impurity species in theprocess gas streams employed in semiconductor manufacturing ispotentially deleterious. Even small levels of impurities on the order ofparts-per-million (ppm) can cause inconsistent electrical properties insemiconductor devices manufactured by deposition techniques usingimpurity-containing gas streams.

It therefore is an object of the present invention to provide a systemfor detection of impurity concentrations in a flowing gas stream, whichcan be usefully employed in semiconductor manufacturing operations.

It is another object of the present invention to provide a detectionsystem of such type, which is capable of providing real-time monitoringof process gas streams, so that immediate correction can be undertakenwhen impurity concentration levels exceed predetermined set pointlimits.

It is a further object of the invention to provide a system fordetecting impurity species in flowing gas streams, which is employed insemiconductor manufacturing operations, and which does not require anyside stream or slip stream sampling for its utilization.

It is a still further object of the invention to provide a system fordetecting impurity concentrations in a flowing gas stream, which isreadily calibrated and has a substantial continuous service life, e.g.,on the order of at least six months.

It is yet a further object of the invention to provide a system fordetecting the calibrated concentration of an impurity species in aflowing gas stream, in a low concentration range, and in a highconcentration range, with quick recovery to normal operation afterexposure to high concentrations of impurity.

Other objects and advantages of the present invention will be more fullyapparent from the ensuing disclosure and appended claims.

SUMMARY OF THE INVENTION

The present invention relates to an in-line detector system, useful forreal-time determination of impurity concentrations in a flowing gasstream.

In one broad aspect, the invention relates to an in-line monitoringsystem for determining the calibrated concentration of an impurityspecies in a flowing gas stream, in a low concentration range below apredetermined concentration value, and in a high concentration rangeabove said predetermined concentration value, said system comprising:

(a) means defining a flow path for a flowing gas stream;

(b) a gas purifier through which gas deriving from the flowing gasstream is flowable to remove impurity species therefrom;

(c) means for sensing the impurity species concentration of gas derivingfrom the flowing gas stream;

(d) means for selectively flowing gas from the flow path through the gaspurifier to yield purified gas depleted in said impurity species;

(e) means for flowing purified gas from said gas purifier to saidsensing means;

(f) means for selectively flowing gas from the flow path to the sensingmeans without passage of the gas through the gas purifier;

(g) means operatively coupled to said sensing means for determining acalibrated equilibrium value of impurity species concentration in theflowing gas stream from impurity species concentration in unpurified gassensed by said sensing means in calibrative relationship to impurityspecies concentration of purified gas sensed by said sensing means;

(h) flow control means operatively coupled with gas flowing means (f)and (g), and arranged to selectively, alternatively and repetitivelyflow gas deriving from the flowing gas stream through gas flowing means(f) to the gas purifier for a first selected period of time t₁, andsubsequently through gas flowing means (g) for a second selected periodof time t₂, in a repeating cycle, wherein in the low concentration rangebelow said predetermined concentration value, t₂ >t₁, and wherein in thehigh concentration range above said predetermined concentration value,t₁ >t₂.

In another aspect of the invention, a method is provided for determiningthe calibrated concentration of an impurity species in a flowing gasstream, in a low concentration range above a predetermined concentrationvalue, and in a high concentration range above said predeterminedconcentration value, said method comprising:

providing an in-line monitoring system comprising:

(a) means defining a flow path for a flowing gas stream;

(b) a gas purifier through which gas deriving from the flowing gasstream is flowable to remove impurity species therefrom;

(c) means for sensing the impurity species concentration of gas derivingfrom the flowing gas stream;

(d) means for selectively flowing gas from the flow path through the gaspurifier to yield purified gas depleted in said impurity species;

(e) means for flowing purified gas from said gas purifier to saidsensing means;

(f) means for selectively flowing gas from the flow path to the sensingmeans without passage of the gas through the gas purifier;

(g) means operatively coupled to said sensing means for determining acalibrated equilibrium value of impurity species concentration in theflowing gas stream from impurity species concentration in unpurified gassensed by said sensing means in calibrative relationship to impurityspecies concentration of purified gas sensed by said sensing means;

(h) flow control means operatively coupled with gas flowing means (f)and (g), and arranged to selectively, alternatively and repetitivelyflow gas deriving from the flowing gas stream through gas flowing means(f) to the gas purifier for a first selected period of time t₁, andsubsequently through gas flowing means (g) for a second selected periodof time t₂, in a repeating cycle, wherein in the low concentration rangebelow said predetermined concentration value, t₂ >t₁, and wherein in thehigh concentration range above said predetermined concentration value,t₁ >t₂ ;

flowing gas from the flowing gas stream selectively from the flow paththrough the gas purifier to yield purified gas depleted in said impurityspecies, alternatingly with flowing gas from the flow path through thegas flowing means (f), and passing the gas to said sensing means; and

selectively operating said flow control means (h) to flow gas throughmeans (f) to the gas purifier for a first selected period of time t₁,and subsequently through gas flowing means (g) for a second period oftime t₂, in a repeating cycle, wherein in the low concentration rangebelow said predetermined concentration value, t₂ >t₁, and wherein in thehigh concentration range above said predetermined concentration value,t₁ >t₂.

In yet another aspect of the present invention, a method is provided fordetermining a calibrated concentration of an impurity species in aflowing gas stream, in a low concentration range below a predeterminedconcentration value, and in a high concentration range above saidpredetermined concentration value, said method comprising:

sensing the impurity species concentration of gas derived from the gasflow stream in a sequential and repetitive sensing operation wherein ina first sensing mode the gas is purified of impurity species prior tosensing thereof and wherein in a second sensing mode the gas isunpurified, and wherein in the first sensing mode and second sensingmode, the sensing mode cycle times are varied in accordance with theimpurity species concentration, so that in said low concentration rangebelow said predetermined concentration value, the second sensing modefrequency is greater than the first sensing mode frequency, and whereinin said high concentration range above said predetermined concentrationvalue, said first sensing mode frequency is greater than the secondsensing mode frequency; and

determining a calibrated equilibrium value of impurity speciesconcentration in the flowing gas stream from sensing concentrations inthe first and second sensing modes.

In a still further aspect, the present invention relates to an in-linemonitoring system for determining a calibrated concentration of animpurity species in a flowing gas stream, in a low concentration rangebelow a predetermined concentration value, and in a high concentrationrange above said predetermined concentration value, said systemcomprising:

a purifier for removing impurity species from gas derived from theflowing gas stream;

a means for sensing the impurity species concentration of gas derivedfrom the gas flow stream in a sequential and repetitive sensingoperation wherein in a first sensing mode the gas is purified ofimpurity species by passage thereof through said purifier prior tosensing thereof and wherein in a second sensing mode the gas isunpurified, and wherein in the first sensing mode and second sensingmode, the sensing mode cycle times are varied in accordance with theimpurity species concentration, so that in said low concentration rangebelow said predetermined concentration value, the second sensing modefrequency is greater than the first sensing mode frequency, and whereinin said high concentration range above said predetermined concentrationvalue, said first sensing mode frequency is greater than the secondsensing mode frequency; and

a means for determining a calibrated equilibrium value of impurityspecies concentration in the flowing gas stream from sensingconcentrations in the first and second sensing modes.

In another aspect, the present invention relates to a method ofdetermining a calibrated concentration of impurity species in a flowinggas stream, wherein gas derived from the gas flow stream isalternatingly sensed as to its impurity species concentration inpurified and unpurified conditions, and said sensing comprisescontacting of gas with a sensor including a binding medium havingirreversible binding affinity for the impurity species, said methodcomprising sensing impurity species concentration of unpurified gas bysaid sensor in a differential mode, wherein sensing time of the sensorin exposure to said unpurified gas is less than 50% of the operatingtime of the sensor, whereby the sensor service life is extended relativeto continuous sensing of unpurified gas.

In yet another aspect, the present invention relates to an in-linemonitoring system for determining a calibrated concentration of impurityspecies in a flowing gas stream, comprising:

a sensor including a binding medium having irreversible binding affinityfor the impurity species, for sensing impurity species concentration ina gas;

means for purifying gas derived from said flowing gas stream to yieldpurified gas;

means for flowing unpurified gas derived from said flowing gas stream tosaid sensor and for flowing purified gas from said gas purifying meansto said sensor, such that said unpurified gas is sensed by said sensorin a differential mode wherein sensing time of the sensor in exposure tosaid unpurified gas is less than 50% of the operating time of thesensor, whereby the sensor service life is extended relative tocontinuous sensing of unpurified gas; and

means for determining a calibrated concentration of impurity species insaid flowing gas stream from impurity species concentrations of saidpurified and unpurified gas sensed by said sensor.

In a related aspect, the present invention relates to an in-linedetector system, comprising:

a purifier unit for gas stream impurity removal; and

means defining a flow passage assembly having an inlet end and an outletend, and constructed and arranged:

for flowing at least a portion of gas from the flowing gas streamthrough the purifier unit to yield impurity- reduced gas;

for flowing impurity-reduced gas to an impurity concentration sensinglocus;

for flowing unpurified gas from the flowing gas stream to an impurityconcentration sensing locus; and

for discharging the impurity-reduced gas and unpurified gas from theoutlet end.

Another related aspect of the invention relates to an in-line detectorsystem whose sensitivity may be conveniently recalibrated periodically,comprising:

a purifier unit for gas stream impurity removal; and

means defining a flow passage assembly having an inlet end and an outletend, and constructed and arranged:

for flowing at least a portion of gas from the flowing gas streamthrough a purifier unit to yield impurity-reduced gas;

for flowing at least a portion of impurity-reduced gas to an impurityconcentration sensing locus;

for flowing at least a portion of the gas stream to an impurity standardlocus which transmits a known impurity concentration to the gas;

for flowing gas from the impurity standard locus to the impurityconcentration sensing locus.

In another related aspect, the present invention relates to an in-linedetector system, comprising:

a purifier unit for removing impurity from gas containing same;

a manifold assembly having an inlet joinable in flow communication tothe flowing gas stream for passage of the flowing gas streamtherethrough, and an outlet for discharging from the manifold assemblygas flowed therethrough, with such manifold assembly defining (i) afirst flow path coupled to the purifier unit for passing gas from theflowing gas stream through the purifier unit to yield impurity-reducedgas, and (ii) a second flow path bypassing the purifier, constructed andarranged such that the manifold assembly derives from the flowing gasstream first flow path and second flow path gas streams, and dischargessame through the outlet of the manifold assembly;

means for (a) sensing gas impurity concentration of impurity-reduced gasdischarged from the purifier unit into the first flow path of themanifold assembly, as a baseline impurity concentration value, (b)sensing gas impurity concentration in the second flow path gas stream,and (c) determining therefrom a baseline-adjusted impurity concentrationvalue for the flowing gas stream.

In a further related aspect, the present invention relates to an in-linedetector system whose sensitivity can be conveniently calibratedperiodically, additionally comprising:

(i) a first flow path coupled to a purifier unit for passing gas from aflowing gas stream through the purifier unit to yield impurity-reducedgas, and subsequently passing the impurity-reduced gas through animpurity standard locus where a known concentration of impurity isimparted; (ii) a second flow path bypassing the impurity standard locus,constructed and arranged such that the manifold assembly derives fromthe flowing gas stream first flow path and second flow path gas streams,and discharges same through the outlet of the manifold assembly;

means for (a) sensing gas impurity concentration of gas discharged fromthe impurity standard locus into the first flow path of the manifoldassembly, as a baseline impurity concentration value, (b) sensing gasimpurity concentration in the second flow path impurity-reduced gasstream, and (c) determining therefrom a standard impurity concentrationvalue which is used to calibrate the detector.

The purifier unit in the broad practice of the invention may be of anysuitable type, but preferably comprises a vessel containing a bed of ascavenger material which is sorptively selective for the impurity in theflowing gas stream.

The manifold assembly may be variously configured, and in one aspect maycomprise a main gas flow conduit to which is joined, in spaced-apartrelationship to one another, the purifier unit and a sensor portcoupleable with suitable impurity concentration sensing means. In thisassembly, a T-shaped conduit member is utilized having a verticallydepending leg joined at a lower end thereof to the main gas flowconduit, and at an upper end thereof to laterally extending arms. Theouter ends of the laterally extending arms are respectively joined tothe purifier and the sensor port, with a switcher valve, e.g., apneumatic valve or an electrically controlled solenoid, disposed at theintersection of the arms and leg of the T-shaped conduit member, toselectively establish flow from the main gas flow conduit through thepurifier unit to the sensor port, or alternatively through the leg andan arm of the T-shaped conduit member to the sensor port.

Alternatively, the manifold assembly may be configured with an inlet gasflow conduit joined to an inlet gas manifold which at its extremities isjoined to respective first and second branch flow conduits, and with thebranch flow conduits joined at their opposite ends to an outlet gasmanifold which in turn communicates with an outlet gas flow conduitattached to the outlet gas manifold. With such manifold assembly, apurifier unit may be disposed in one of the manifolds or branch flowconduits upstream of a first impurity sensor, while a second impuritysensor is disposed either upstream of the purifier unit, or else in anopposite portion of the manifold assembly through which gas is notflowed to the purifier unit. In this manner the respective impuritysensors constitute a reference sensor (downstream from the purifierunit) and a sample sensor (the sensor deployed in the portion of themanifold assembly through which the gas stream is not flowed to thepurifier unit), and the sensing of the reference and sample sensors maybe employed to determine a baseline-adjusted concentration value for theimpurity in the gas stream.

In a preferred aspect, the impurity concentration sensing means, whenwater is the impurity species, comprises a hygrometric or alternativelya piezoelectric-based concentration sensor. For non-aqueous impurityspecies, a piezoelectric-based device preferably is employed, such as asurface acoustical wave (SAW) device.

Another aspect of the invention relates to SAW devices comprisingspecific affinity coatings on the piezoelectric substrate of the device,as specific to particular impurity gas species.

Other aspects of the invention relate to in-line detector systems of thetype broadly described above, as associated with sensors and signalgenerating and processing means for determining a baseline-adjustedconcentration value of the impurity in the flowing gas stream.

Other aspects of the invention include appertaining methodology fordetermining impurity concentration in a flowing gas stream by in-linedetection, wherein gas from the flowing gas stream is purified andsensed to determine its impurity concentration, together with sensing ofthe impurity concentration of gas from the flowing gas stream which hasnot been purified, and a calibrated, or baseline-adjusted, concentrationvalue is determined for the impurity in the flowing gas stream.

Other aspects and features of the invention will be more fully apparentfrom the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a detector system according toone embodiment of the present invention.

FIG. 2 is a perspective view of a detector system assembly according toone aspect of the present invention, such as may be usefully employed inthe practice of the invention to sense impurity concentrations inflowing gas streams.

FIG. 3 is a perspective view of the detector system assembly of FIG. 2,schematically shown as being mounted in a modular housing.

FIG. 4 is a perspective view of a detector that may be periodicallyrecalibrated at two concentrations, a baseline concentration and awell-characterized, non-zero value of impurity concentration in aflowing gas stream, having a means for calibration at the low, baselineimpurity concentration and at the higher well-characterized impurityconcentration.

FIG. 5 is a perspective view of a detector that may be periodicallyrecalibrated at many concentrations, a baseline concentration and manywell-characterized, non-zero values of impurity concentration in aflowing gas stream, having a means for calibration at the low, baselineimpurity concentration and at the higher well-characterized impurityconcentrations.

FIG. 6 is a schematic representation of a permeation tube device whichmay be used as a calibration standard for gas impurity sensors employedin the practice of the present invention.

FIG. 7 shows the output signal versus time for a hygrometer sensorexposed to 1.6 ppm moisture in a 2 liter per minute inert gas streamalternately with a purified ("zero") 2 liter per minute inert gasstream, and then to 0.3 ppm moisture in a 2 liter per minute inert gasstream alternately with the zero gas stream.

FIG. 8 shows a close-up of the data from FIG. 7, for two cycles at the1.6 ppm moisture level.

FIG. 9 shows a calibration curve for a hygrometer sensor, made over therange of 300 to 3000 ppb, by which the change in signal can beextrapolated to the equilibrium concentration.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

The present invention overcomes the deficiencies of prior art gasmonitoring systems, as described in the "Background of the Invention"section hereof, by the provision of an in-line gas impurity detector andmonitoring system which is specifically adaptable for use in themanufacture of semiconductor devices including vapor deposition-basedprocesses.

The detector system of the present invention enables the concentrationof critical impurities in a flowing gas stream to be measured at lowlevels (e.g., part-per-billion concentrations), thereby permittingimpurity concentrations in the flowing gas stream to be controllablymaintained at suitably low levels for efficient semiconductor devicemanufacturing, and may be usefully employed in combination with meansfor initiating purification treatment of the flowing gas stream whenimpurities therein exceed acceptable set point limits. The detectorsystem of the present invention can monitor concentration of an impurityspecies in a flowing gas stream, in a low concentration range, and in ahigh concentration range, with quick recovery to normal operation afterexposure to high concentrations of impurity.

Since the detection system of the present invention is an in-linesystem, there is no wastage of the process gas, or hazards occasioned byits diversion, treatment, and disposal, such as is the case in impurityconcentration monitoring systems in which side streams are separatedfrom the main gas flow stream and passed to remote concentration sensingmeans.

Because the detection system of the present invention is capable ofbeing calibrated both as to the sensor's baseline (or "zero point") andat a known impurity concentration preferably typical of the impurityconcentrations that might be found in the flowing process gas stream,the detection system can use impurity sensors that are readily availableand economically feasible, despite the fact that these sensors mayundergo some drift with time in both baseline and response curve.

As a result, the in-line detector systems of the present inventionpermit consistent delivery of high purity gases to be achieved, withdirect, real-time measurement of critical impurity concentrations inprocess gas streams. In conjunction with the use of such detectionsystems, the deployment of in-line, point-of-use purification systemsenables gas purification to be correctively undertaken so thatpredetermined purity set point limits for the flowing gas stream arereadily and continuously maintained.

Examples of gas purification systems which may be usefully employed inconjunction with the in-line detector systems of the present invention,include the purifier apparatus, compositions (scavengers orchemisorbents), and methods disclosed and claimed in U.S. Pat. Nos.:4,761,395 (composition for purification of arsine, phosphine, ammonia,and inert gases); U.S. Pat. No. 4,853,148 (hydrogen halidepurification); U.S. Pat. No. 4,797,227 (hydrogen selenide purification);U.S. Pat. No. 4,781,900 (method of purifying arsine, phosphine, ammonia,and inert gases); U.S. Pat. No. 4,950,419 (composition for inert gaspurification); U.S. Pat. No. 5,015,411 (inert gas purification method);U.S. Pat. No. 4,865,822 (hydrogen selenide purification method); andU.S. Pat. No. 4,925,646 (hydrogen halide purification method); U.S. Pat.No. 5,057,242 (composition for purifying chlorosilanes); U.S. Pat. No.5,094,830 (chlorosilanes purification method); applicant's co-pendingU.S. Pat. application Ser. No. 08/084,414 (purification of inert andhydride gases); as well as the purifier vessel apparatus disclosed andclaimed in U.S. Pat. Nos. 4,723,967 and 4,738,693, all of which herebyare incorporated herein by reference.

The in-line detector systems of the present invention may be utilizedwith any suitable specific gas impurity concentration sensors, whichprovide output indicative of impurity concentration in the gas stream.Such sensors may be hygrometric (in the case of monitored aqueouscontaminants such as water), spectrophotometric (based on transmissivityor reflectivity of radiation impinged on the gas stream), piezoelectric,colorimetric, etc., in character or may otherwise incorporate anysuitable means, method, or modality of operation, as desired, forquantitation of the selected impurity species in the flowing gas streambeing monitored. The active medium of the sensor typically is a mediumhaving a binding affinity for the species of interest, such as aphysical adsorbent material.

Generally, in semiconductor manufacturing operations, the most criticalimpurity species is water vapor, and the presence of water in the gasstream often is indicative of atmospheric contamination of the processsystem. Accordingly, the invention will be illustratively describedhereinafter primarily with reference to detection of water as theimpurity species of interest. It will be recognized however, that suchfocus is for descriptive purposes only and that the invention is broadlypracticable in monitoring of any other impurity species, for whichsuitably sensitive sensors exist. Illustrative examples of impurityspecies and corresponding potentially useful sensor media include, butare not limited to: water/alumina (hygrometer), HCl/alumina(hygrometer), water/polyamine (piezoelectric detector), water/P₂ O₅(electrolytic detector; has an irreversible component), ammonia/alumina(hygrometer), methanol/alumina (hygrometer), oxygen/tin oxide (solidstate electrode).

While the specific structure and function of the in-line detectorsystems may be widely varied within the broad scope of the presentinvention, such in-line detection systems must meet various functionalcriteria, as set out below.

First, the detector system must be non-contaminating in character, withrespect to the gas stream being processed. Since the flowing gas streamafter its monitoring (and verification of suitably low impurityconcentration therein) is flowed to the deposition reactor or otherlocus of use, any contaminants deriving from the detector system willsubsequently be distributed throughout the process system. This may havea significant and deleterious effect on the products being manufactured.Accordingly, any impurities introduced from the in-line detector systemitself should be suitably low, e.g., in the part-per-billion (ppb) rangeor lower.

Thus, the detector system must be mechanically tight and leak-free incharacter. This requirement dictates the use of correspondingly suitablematerials of construction in the detector system, with the parts andcomponents of the detector system having a high finish on those partsand components which are in contact with the gas stream, and with allseals of the detector being of a face seal, leak-tight character.

A preferred material of construction is stainless steel of suitably highfinish quality. If any particulates are generated in the use andoperation of the detector, particle filters may be required componentsof the system.

In addition, the detector system should accurately measure the criticalimpurities in the process stream. Process gas streams employed in thesemiconductor manufacturing industry typically contain more than oneimpurity, and it would be highly advantageous to accurately measure theconcentration of each of such impurities in the gas stream.

As a practical consideration, however, there does not exist a singlesensor which is able to measure all possible impurities. Monitoring allimpurities of interest would therefore require a large multiplicity ofsensor devices, which would in turn unduly complicate the design andoperation of the detector system.

Accordingly, in multicomponent impurity-containing gas streams, it ispreferable to select a single impurity species and monitor same,particularly where the impurity species monitored is a limiting or mostcritical impurity, or where the specific impurity species isquantitatively correlative with other impurity species present in themulticomponent impurity-containing gas stream.

As indicated hereinabove, a critical impurity in semiconductormanufacturing operations is water, and a variety of water sensors isreadily available. By contrast, oxygen, while also an important impurityspecies, is frequently a poor choice for process gas monitoringpurposes, since oxygen sensors are poisoned by many of the(semiconductor manufacturing) gas streams of interest, so that as apractical matter, viable sensors are not available. In this respect, itshould be borne in mind that the sensors required are preferablysub-part-per-million (ppm) level sensors.

It will be recognized that in the instance where the impurity species isa potential poison to the sensor element, the differential sensing modeof the present invention may afford a manner of extending the sensorlife to periods of time which in the absence of such operation would bepractically unobtainable.

Thus, the detector system should have the requisite sensitivity fordetection of gas impurities, preferably on the level ofparts-per-million and most preferably on the level of parts-per-billion.

Further, the detector system should desirably be stable for substantialperiods of time, e.g., at least six months, and preferably on the orderof one year or more, without recalibration continually being required.Thus, the sensors used in the detector should be of a non-driftingcharacter, or if drift is necessarily present, means should be providedto self-correct the concentration measurement so that stable andconsistent quantitative operation is achieved, with respect to theimpurity concentration in the gas stream being monitored. Such means ofself-correction should include calibration of both baseline andsensitivity to known impurity concentration, since both baseline andresponse curve may drift with time.

Additionally, the cost of the detector system should be suitably low toensure ready commercial deployment, with economic, readily availablesensor devices being utilizable in the detector system.

The foregoing criteria are accommodated in the broad practice of thepresent invention by the provision of a detector system in which gasfrom the flowing gas stream (either a portion of such stream, or theentire stream itself) is passed through a purifier in which the impuritybeing monitored is substantially completely removed from the gas. Theimpurity concentration of the resulting purified gas is then sensed toprovide a baseline concentration sensing value. Contemporaneously, gasfrom the flowing gas stream, which is not purified by the aforementionedpurifier unit, is subjected to impurity concentration sensing means.This provides a sensed concentration value which is employed, togetherwith the impurity concentration sensing valve for the purified gas, toprovide a calibrated value of impurity concentration for the flowing gasstream.

The gas passed through the purifier and subsequently sensed for impurityconcentration, is discharged from the detector system in the flowing gasstream, and the sensed gas which has not been purified likewise isdischarged from the detector system in the flowing gas stream. By thisarrangement, none of the influent flowing gas stream passed through thedetector system is diverted to waste, or otherwise requires finaldisposition as a result of its being monitored for impurity content (theonly exception is the impurity species which is removed from gas in thepurifier, e.g., which is typically present in the gas flowed to thepurifier at a low concentration, e.g., less than about 1,000 ppm andmore typically less than 100 ppm).

Correspondingly, the sensor which is employed with the in-line detectorsystem should have a suitably low detection limit, preferably sub-ppmlevels, and more preferably down to 100 parts-per-billion (ppb), orlower.

Among the various types of sensor devices described hereinabove(hygrometric, spectrophotometric, etc.), a preferred sensor is ofpiezoelectric-type, in which the characteristics of the piezoelectricsurface thereof are altered by the presence and any change inconcentration of the impurity species being monitored in the detectorsystem.

A particularly preferred piezoelectric device comprises a surfaceacoustical wave (SAW) device. SAW devices are piezoelectric electroniccomponents which traditionally have been used as narrow band frequencyfilters, e.g., frequency-determining elements in high frequency controlapplications. Such devices operate by passing a signal across apiezoelectric thin planar substrate as an acoustic wave. The acousticwave is created by imposing an AC electrical signal on a metallizedinterdigital electrode which is plated on the surface of the substrateat one end. This acoustic wave is transmitted across the substrate to asymmetrically formed metallized interdigital electrode (transducer) atthe opposite end. The receiving transducer converts the acoustic signalback to an electric signal. The electrical characteristics of SAWdevices can be tailored to specific application by varying the fingerspacing of the interdigital transducers, the space between transducers,and the thickness of the substrate, as is well known to those skilled inthe art, to control the frequency, propagation delay, and acoustic wavemode of the signal transmitted across the device.

For monitoring low moisture levels, a preferred sensor type ishygrometric, wherein a property such as capacitance of a material whichhas a high sorptive affinity for water is monitored. Such hygrometricsensors have as the active sensing material substances such as ultra-dryalumina, which adsorb water with high affinity. Other contaminantspecies which may be sensed by such hygrometric sensors include polarcompounds such as ammonia, methanol, etc.

Regardless of the specific type of impurity concentration sensoremployed in the broad practice of the present invention, the sensorshould possess the sensitivity to measure the concentration of thecritical impurities at sufficiently low levels consistent with the highpurity character of the gas streams being monitored by the detectionsystem.

In most semiconductor processing applications of the in-line monitor,the sensing means preferably has extremely high sensitivity, because theamount of moisture or other contaminant species present in the gasstream is typically quite low under normal operating conditions, e.g., afew parts per billion. Examples of highly responsive sensors includecapacitive hygrometers, which measure trace levels of moisture bymonitoring the capacitance of a material such as ultra-dry alumina,which has an extremely high affinity for water. The alumina or otheractive sensor material, in contact with the gas stream, binds tracelevels of contaminant it encounters, manifesting as a change incapacitance of the sensor material. Piezoelectric detectors bind thecontaminant to a high affinity layer on the surface of the detector,manifesting as a change in the frequency of the vibrations of the filmbecause of the change in its weight.

Many of the most highly sensitive detectors operate by interacting withthe analyte via sorption, physical association, aggregation, or otherbinding process, and do so with high affinity. When such a sensormaterial, interacting with the analyte by a physical sorption process,encounters high concentrations of analyte, e.g., moisture or othermonitored contaminant, as it may do in the event of a serious leak orcompromise of the gas handling system, it may not be able toreequilibrate to the lower level of contaminant sufficiently quickly forconvenient operation. A long "dry-down" period may be required, duringwhich the usefulness of the in-line-monitoring system is seriouslycompromised. This limitation can be a major problem if high contaminantlevels are encountered in a gas stream during an "upset," e.g., a majorleakage event, compromise of the gas handling system, inadvertentflowing of the wrong gas or vapor phase reactant into the process, orother abnormal situation in which the sensor material encounters a largebolus of the analyte (contaminant being monitored for).

In impurity-sensitive vapor-phase operations, e.g. semiconductor deviceprocessing or vapor phase coating of fiber-optic filaments, protectionof the process from exposure to the high contaminant concentrations thatcould be encountered in these "upset" events is crucial and is animportant function of the in-line monitoring system. During an upset,the concentrations of moisture or other contaminants can be very high.Semiconductor processors have indicated that they want to be able toestimate moisture levels to 20 part per million (ppm), which issignificantly higher than the few parts per billion (ppb) levelsencountered in a process that is running smoothly. If the highlysensitive, sorption-based sensors described above reach equilibrium atthe higher contaminant level, they may take hours to days of "dry-down"for the output signal to recover to the baseline level.

The solution to the problem of slow recovery/response time is to operatethe in-line monitor in a differential mode, in situations where highconcentrations of contaminant are encountered. In the normal mode, thesensor is exposed the majority of the time to the sample gas (the gasstream whose impurity level is being monitored). The characteristics ofthe sensor in this normal mode are high sensitivity and an output signalthat reflects the full equilibration of the sensor with the gas stream.In the differential mode, the sensor is left in the purifier mode, thatis, it is exposed the majority of the time to the gas stream that haspassed through the purifier module. For a short period of time, theby-pass valve is opened. The sensor takes a short look at the sample gasstream. The by-pass valve is then closed and the system is allowed torecover. The cycle then repeats. Using the output signals from the briefexposures to the high contaminant sample gas stream, the system thenextrapolates the change in contaminant concentration to the equilibriumvalue.

A sensor material which interacts with the analyte by physical sorption,e.g., a hygrometer or SAW device, behaves like a sponge. If the spongeis left in a spray of water for a long time, switching to a drying gaswill require a long period of time to dry out the sponge. If the spongeis sprayed very briefly and returned to the dry gas stream, recovery isfast.

The key to operation in the differential mode is to allow surfaceequilibration but not bulk equilibration of the sensing medium.Measurements are made at dynamic contact equilibrium rather than atstatic equilibrium of the sensing medium, where the dynamic contactequilibrium is a surface equilibrium phenomenon, and the staticequilibrium is a bulk equilibration condition. The in-line monitor isconfigured so that it is possible to selectively, alternatively andrepetitively flow gas deriving from the gas stream being monitoredthrough the gas purifier module for a first selected period of time t₁,and subsequently through the contaminant sensing module for a secondselected period of time t₂, in a repeating cycle. The time t₁ is chosensuch that exposure of the sensor to the purified gas is adequate topermit accurate baseline concentration calibration, with some baselinedrift being allowable in practice, since impurity concentrations aremeasured by difference. Time t₂ is chosen to allow dynamic contactequilibrium to occur but not static equilibrium, and the detector cantherefore recover much more quickly to equilibrium with thelow-contaminant concentration gas stream, without changes in itsresponse curve. The actual concentration of contaminant in the gasstream being monitored is derived by extrapolating the change incontaminant concentration to the equilibrium value.

The cycle times will depend on the nature of the contaminant species andthe sensor, the sorptive affinity of the sensing or binding medium forthe contaminant species, the gas flow rate, the contact time with thesensing medium, and other system parameters. For a sensor comprising abinding medium with high sorptive affinity for the contaminant species,the binding medium would typically be exposed to the sample gas for atime period many-fold less than the exposure time to the purified gasstream (or "zero gas"), on the order of 5-100 fold less. For example,the binding medium could be exposed to the sample gas for time for a fewseconds to minutes, e.g., 10-60 seconds, followed by exposure to thezero gas for a time period on the order of a few to tens of minutes,e.g. 3-30 minutes.

Referring now to the drawings, FIG. 1 shows a schematic diagram of anin-line detection system 100, comprising an in-line detector assembly102. Conduit 104 is joined to the detector assembly and conveys the highpurity gas stream to the assembly for monitoring therein of theconcentration of critical impurity species. Monitored gas is dischargedfrom the detector assembly in line 103 and is passed to flow controller105, which is coupled to the detector assembly in signal transmittingrelationship by means of flow control signal line 107. If the monitoringof the critical impurity species reveals that the concentration is aboveset point limits, the flowing gas stream is diverted by the flowcontroller 105 into bypass line 107a and flows into gas scrubbing orpurification complex 109 in which the impurity species is removed tobelow the maximum set point concentration value. The resulting scrubbedgas then is passed in line 111 to the process gas delivery conduit 113and is discharged into the vapor-phase processing complex 115.

If the concentration of the critical impurity species monitored bydetector assembly 102 is within allowable set point limits, the flowinggas stream in line 103 is passed by the controller 105 to delivery line113 for passage to the downstream, end use complex 115. Complex 115 may,for example, comprise a chemical vapor deposition reactor or othersuitable downstream processing equipment.

Joined to the in-line detector assembly 102 in signal transmitting andreceiving relationship, via signal line 106, is a controller 108. Thiscontroller may include optoelectronic converters, digital/analogcircuitry, etc., by means of which the sensing of impurity species bythe in-line detector assembly 102 is convertible to a processing signal.This processing signal is transmitted by signal transmitting means 110to a digital computer 112 comprising central processing unit 114 andmonitor or display 116. Alternatively, the digital computer 112 may bereplaced by microprocessor, programmable logic device, or othercalculational means which are incorporated in or otherwise integratedwith the controller 108.

In operation of the FIG. 1 system, the in-line detector assembly 102,coupled with a suitable sensor or sensors, is arranged to senseconcentration of critical impurity species in the flowing gas stream, aswell as to sense the critical impurity species concentration of gas fromthe flowing gas stream which has been subjected to purification.

These respective concentration sensings then are passed by signaltransmitting means 106 to controller 108, and converted to the requisiteform (of a processing signal) for computational purposes. The processingsignal is passed by processing signal transmitting means 110 to digitalcomputer 112 for determination of an on-line, real-time concentrationvalue, as corrected (normalized) by the purified gas (impurityconcentration sensing) valve.

The normalization (correction) of sensing of critical impurity speciesin the flowing gas stream, by correspondingly sensing the criticalimpurity species in a purified stream of the gas from the bulk flowinggas stream passed to the in-line detector, is a critical aspect of thepresent invention, which eliminates problems that may occur in sustainedoperation of the detector system as a result of sensor "drift".

Thus, with the high sensitivities desired of the sensors in the broadpractice of the present invention, e.g., below 1000 parts-per-million,preferably below 100 parts-per-million, more preferably below 10parts-per-million, and most preferably below one part-per-million, thesensors invariably have a tendency to drift (change in accuracy) withtime. This is particularly true in continuous operation ornear-continuous operation semiconductor manufacturing plants, in whichgas is flowed through the detector system over sustained periods oftime. Both the baseline signal and the response curve may drift overtime. Such alteration of sensitivity and accuracy of the sensors withtime, unless continuously corrected, can insidiously lead to gasimpurity concentrations exceeding proper set point limits without beingidentified as "out of spec". This in turn may cause excessive amounts ofimpurities to be present in layers and films deposited on substrates inthe subsequent vapor-phase processing operations. The resultinginclusions of gross amounts of impurity species in product devices whichcan render them deficient or even useless for their intended purpose.

The baseline drift problems associated with high sensitivity sensor(s)in the detection system are self-corrected in the practice of thepresent invention, by sensing the concentration of impurity species ingas derived from the bulk flow stream, after it has been purified in anin-line fashion, with such purified gas impurity sensing then beingemployed as a baseline corrective value or condition for the sensing ofimpurities in the (unpurified) bulk flow stream.

In conjunction with self-correction of the sensor baseline, any drift inthe response curve of the sensor in the detection system can also beself-corrected in the practice of the present invention, by theprovision of one or more known concentrations of impurity species to thesensor, by means of which the sensor's response curve may bere-calculated. Recalibration of both baseline and response curve may beperformed periodically without taking the sensor off-line.

By "in-line detection" as used herein is meant that the flowing gasstream of interest is sensed as to its impurity concentration, and alsois at least in part purified to provide a baseline impurityconcentration sensing, with both sensings being carried out in the samegeneral locus, and with any gas in circulation loops employed forsensing purposes being redirected into the bulk gas flow stream forultimate delivery to the downstream processing facility, e.g., chemicalvapor deposition reactor.

Referring now to FIG. 2, there is shown a perspective view of an in-linedetector assembly 200, comprising an inlet gas flow conduit 202 with aninlet fitting 204 at one end, and connected in flow relationship to aoutlet gas flow conduit 205 with an outlet fitting 206. A purifiervessel 208 is joined in inflow relationship to the main conduit 202 bymeans of purifier feed conduit 210.

The in-line detector assembly 200, by means of the respective inlet anddischarge fittings 204 and 206, may be coupled in an "in-line" fashionto a gas stream which ultimately is passed to a chemical vapordeposition (CVD) reactor, or other vapor-phase processing apparatus.

As appearing subsequently herein, identifications of a component of thedetector system being joined "in inflow relationship" to anothercomponent of the system, means that the components are constructed andarranged so that the gas stream flows from the first-mentioned componentto the second-mentioned component. Correspondingly, the identificationof a detector system component as being joined "in outflow relationship"to another system component, means that the respective components areconstructed and arranged so that the gas stream from the first-mentionedcomponent is discharged (through any suitable flow communication means)to the second-mentioned component of the system.

Downstream of the purifier feed conduit 210, and in proximity to thedischarge fitting 206, is provided an impurity sensor port 212, with anupper fitting 214. By means of the upper fitting 214, the sensor portmay be coupled to a suitable sensor, such as a hygrometric sensor forwater vapor detection, or a SAW device for detection of water, nitrogenoxides, hydrogen sulfide, or other critical impurity of interest. Thesensor port 212 is joined to the outlet fitting 206 by means of gas flowconduit 216.

The purifier unit 208, depending on the impurity species, may comprise afluid-tight vessel filled with a bed of a suitable scavenger material.The scavenger material may for example comprise a scavenger of a type asdisclosed in U.S. Pat. Nos. 4,761,395; 4,853,148; 4,797,227, in apurifier vessel 219 of the type disclosed and claimed in U.S. Pat. No.4,723,967 or U.S. Pat. No. 4,738,693. The purifier vessel 208 maysuitably be provided, if desired, with a pressure transducer port 218,for measurement of pressure in the purifier unit. The purifier vessel208 also features an outlet port 220, by means of which purified gas isdischarged from the purifier unit.

The detector assembly shown in FIG. 2 further comprises a manifold 222interconnecting the purifier unit 208 with the sensor port 212, by meansof lateral conduits 224 and 226. Lateral conduit 226 also is joined tothe inlet flow conduit 202 by means of manifold 222. Disposed in themanifold 222, at the locus of intersection of the conduits 224, 226, and202, is a switcher valve 230. This valve preferably is an automaticcontrol-type valve which is selectively switchable between (1) a firstposition for effecting flow of gas through the manifold, in conduits 224and 226 thereof, from the purifier unit 208 to the sensor port 212, and(2) a second position for effecting flow of gas to the manifold from themain flow conduit 202, and through lateral conduit 226 to the sensorport 212.

Disposed within the purifier 208 (and not shown) are two flowrestrictors, whose role is to retain the purifier's scavenger bed andregulate the pressure drop of the gas flow pathway through the purifier.These flow restrictors may be of any suitable type which are adaptableto provide the flow restriction and pressure drop characteristicsnecessary in the detector system. The flow restriction characteristicsof these flow restrictions may include different pressure drops, as inthe illustrative detector system herein described. Alternatively, insome instances it may be desirable for the flow restrictioncharacteristics of the flow restrictors to be the same. The flowrestrictors preferably comprise disk-shaped elements, or "frits,"constructed of sintered metal, porous ceramic, or other flow-permeablemedium which is compatible with the gas stream constituents, and isefficacious for its intended purpose in terms of pressure drop and flowrestriction character.

Preferred restrictor elements include stainless steel frits which arecommercially available from Mott Corporation (Farmington, Conn.). Suchelements are available in the form of 5-100 micron average pore sizedisks.

In operation of the system shown in FIG. 2, the inlet fitting 204 of themain gas flow conduit 202 may be coupled to piping or tubing throughwhich the bulk gas stream is flowed to the detector system, and thedischarge fitting 206 of such conduit is likewise coupled to suitablepiping or tubing for the delivery of the bulk gas flow stream to thedownstream processing apparatus. The back-pressure created by thepurifier scavenger resin bed and the flow restrictors that retainscavenger resin bed in place causes some of the gas to flow through thepurifier feed conduit 210 and some to continue to flow through conduit202 of manifold 222, depending on the setting of the switcher valve 230.The switcher valve suitably is coupled to automatic controller means(not shown), such as a microprocessor-controlled timer actuator for thevalve. The switcher valve may be of any suitable type; preferred typesinclude pneumatic valves and electrically controlled solenoids.

The flow restrictors within the purifier provide containment of thescavenger or other sorbent material which is provided in the form of abed in purifier unit 208. As a result of gas flow through the bed ofscavenger or sorbent material in the purifier unit, particulates mayotherwise be susceptible of migrating into feed conduit 210 or manifoldconduit 224, if the restrictors were not present. In addition, the flowrestrictors provide back pressure for the purifier loop defined by feedconduit 210, purifier unit 208, discharge port 220 and conduits 224 and226 of manifold 222.

When the switcher valve 230 is selectively positioned in a firstposition, gas from the main gas flow stream in conduit 202 flows throughpurifier unit feed conduit 210 into purifier unit 208 where the impurityspecies is removed from the gas to produce an impurity-reduced gasstream. This purified gas stream is discharged from the purifier unit indischarge port 220, and passes through conduits 224 and 226 to thesensor port 212 for sensing by a sensor (not shown) which is operativelyjoined to the sensor port by means of coupling 214.

If the switcher valve 230 is switched to a second position, the gasflowing in conduit 202 will bypass the purifier loop and gas will flowthrough conduits 202 and 226 of the manifold 222, and enter the sensorport 212, so that sensing of the impurity concentration in the gasstream can be effected. Regardless of whether the gas entering thesensor port 212 passes through the purifier loop, or passes throughmanifold conduits 202 and 226, the gas is discharged therefrom in outletconduit 205 for discharge through the outlet fitting 206 into theconnecting piping or tubing (not shown) and delivery to the downstreamprocessing apparatus.

By this arrangement, all of the gas flows in the detector assembly canbe manipulated by a single valve. The sensor port 212 may be coupledwith a suitable sensor, e.g., an alumina hygrometer sensor, by means ofsensor coupling 214.

By means of the detector assembly in FIG. 2, the monitoring of gasimpurity concentration is carried out with no waste stream. All flowsfrom the main flow conduit passing to the sensor port are returned tothe main gas flow stream with no loss. Further, the detector assemblycomprises a purifier unit which provides a non-drifting zero measurementpoint. As a result, it is possible to continuously correct the sensor inthe course of operation of the detector assembly, yielding a stablesensing system for extended periods of time. In addition, since theimpurity-containing gas from the main gas flow stream, as well as theimpurity-reduced gas yielded by the purifier unit, are passedsequentially to a common sensor port, the utilization of a single sensoris facilitated, which obviates the problems associated with differingdrifts of accuracy and sensitivity when multiple sensors are employed.

While the present invention preferably is practiced by using a singlesensor in connection with the in-line detection assembly of theinvention, as for example is accommodated by the detector assembly shownin FIG. 2, the invention may also be carried out with multiple sensors,as will be described more fully hereinafter. It is to be recognized thatthe use of multiple sensors, due to the aforementioned drift phenomena,may require periodic recalibration in order to maintain accurate andreliable monitoring of gas impurities in gas streams flowed through thedetector assembly.

Referring now to FIG. 3, a detector assembly 300 of the type shown inFIG. 2 is shown as being constructed in a unitary housing 310, which maybe of block-like form as shown. Inlet port 304 and outlet port 306 arebolted to the main housing 310 by flanges 305 and 307 respectively.Purifier 318 extends exteriorly of the housing 300. Boss 330 holds thesensor in place within housing 310, and from boss 330 extend electricalleads 331 which carry the signal from the sensor to a signal displayingmeans or to a controller unit which responds to the signal. Port 332 isan air supply inlet to provide compressed air to operate an air-actuatedvalve (not shown; corresponds to valve 230 in FIG. 2) within housing300. By this arrangement, the overall detector assembly 300 represents aunitary, easily mounted structure, which is readily deployed, in-line,in a bulk gas flow stream for detection of impurity concentrationstherein.

The fittings 304 and 306 may be any of a number of suitable typesincluding VCR, VCO or Swagelok, depending upon the requirements of theapplication the gas flow is being employed in. Because flanges 305 and307 are used as means for connecting the fittings to the housing 310,great flexibility in selection of off-the-shelf fittings is possiblewhile conserving all the key features of the in-line detector assembly.

FIG. 4 is a perspective view of a detector system 400 according toanother embodiment of the present invention. This embodiment providescalibration of the sensor baseline and calibration of the detectorsensitivity at a known concentration of impurity species, provided thegas stream flow rate is known and constant.

As illustrated, the detector assembly 400 comprises a housing 410 withinwhich are mounted valves, gas flow conduits and sensor and above whichextend an impurity source 450 and a purifier 408. In the normal sensingmode of operation, valves 421 and 422 are in the open position. The gasflow stream enters the detector assembly at entry port 404 and flowsthrough the detector assembly as follows: through gas flow conduit 405,vertical gas flow conduit 407, open valve 421, gas flow conduit 417, gasflow conduit 409, gas flow conduit 411, open valve 422, gas flow conduit413, gas flow conduit 415, passing into the space around sensor 412 andcontacting the sensor, outlet port 406. Sensor 412 produces a signal inresponse to impurity and is coupled to a means for displaying the signalor to a controller unit which processes the signal.

In the baseline calibration mode, valve 421 is in the open position andvalve 422 is closed. The gas flow stream enters the detector assembly atentry port 404 and flows through the detector assembly as follows:through gas flow conduit 405, vertical gas flow conduit 407, open valve421, gas flow conduit 417, gas flow conduit 409, gas flow conduit 411,into the space above frit 463, through frit 463 and into purifierscavenger bed 408 where impurity is removed to baseline levels, throughfrit 464 and into gas flow conduit 413, gas flow conduit 415, passinginto the space around sensor 412 and contacting the sensor, outlet port406.

In the high point calibration mode, valve 421 is in the closed positionand valve 422 is open. The gas flow stream enters the detector assemblyat entry port 404 and flows through the detector assembly as follows:through gas flow conduit 405, vertical gas flow conduit 407, into thespace below frit 461, through frit 461, through an impurity source bed450 where the gas flow stream attains a known impurity concentration,through frit 462 and into the space above frit 462, down gas flowconduit 403 through check valve, through gas flow conduit 417, gas flowconduit 409, gas flow conduit 411, through open valve 422 and into gasflow conduit 413, gas flow conduit 415, passing into the space aroundsensor 412 and contacting the sensor, outlet port 406.

In the baseline calibration mode, the impurity is removed to a baselineor "zero-point" level by the purifier 408, which may be of the typedescribed above for FIG. 2. The signal obtained in this mode cantherefore be used to correct for any drift in baseline. The valuedobtained in the high point calibration mode can likewise be used tocorrect for any drift in the slope of the response curve of the sensor.

The impurity source bed 450 which is used to impart a knownconcentration of impurity to the gas flow stream for purposes of highpoint calibration must be capable of rapidly equilibrating with the gasflow stream to impart to it an impurity concentration that is a knownfunction of temperature that does not measurably change over the usefullife of the in-line detector. In addition, for calibration to be useful,the impurity concentration imparted by the impurity source bed must bewithin the range of impurity concentrations that are being measured bythe in-line detector in its normal sensing mode. The impurity source bedmust be chosen in the context of the nature of the impurity beingdetected.

In the case of water impurity, which is key to many manufacturingprocesses including semiconductor device fabrication, the zeolitemolecular sieves are suitable impurity source bed materials. The zeolitemolecular sieves have high water capacities, in the range of tens ofpercents by weight, and, at temperatures of interest, are in equilibriumwith suitably low vapor pressures of water. The water vapor pressures inequilibrium with zeolite molecular sieves that are loaded with knownamounts of water can be found by consulting widely available tables ofequilibrium isotherms. These isotherms are available over a wide rangeof potentially useful temperatures, e.g. from -20° C. to 100° C. -200°C. (see Davison Molecular Sieves Adsorption Equilibria, ProductLiterature from W. R. Grace & Co., Davison Chemical Division). Tabulatedbelow are examples of suitable zeolite molecular sieve beds forcalibration of water impurity measurement in semiconductor processinggas flow streams. Equilibrium water vapor pressures were obtained fromadsorption isotherms for each zeolite at 25° C.

    ______________________________________                                                      4%       8%     12%                                             Water Loading (wt %)                                                                          Equilibrium H.sub.2 O V.P.                                    Zeolite Type    (mm Hg at 25° C.)                                      ______________________________________                                        3A              0.004      0.030  0.066                                       4A              0.002      0.018  0.045                                       5A              0.001      0.008  0.050                                       ______________________________________                                    

As can be seen from this table, these zeolite molecular sieve beds canprovide impurity concentrations in the range of interest. In addition,the amount of water loaded into the zeolite is large compared with theamount that the zeolite would either take up from or contribute to a gasflow stream that had typical water impurity levels of 1-50 ppm. As anexample, consider a gas flow stream containing 10 ppm water flowing overa 10 gram impurity source bed comprising Zeolite 4A with 8% watercontent by weight. The water vapor pressure in equilibrium with this bedis 0.018 mm Hg, or, at atmospheric pressure (760 mm Hg), 23 ppm. If 30liters of gas passes over this Zeolite 4A and rapidly equilibrates withit, the amount of water lost from the zeolite would be ≦3×10⁻⁴ g, a verysmall amount compared with the 0.8 grams of water originally present.

If, on the other hand, 30 liters of a gas flow stream containing 40 ppmwater impurity flowed over and rapidly equilibrated with the samezeolite bed, the amount of water added to the zeolite would be >0.4×10⁻³gram. Thus the ability of the zeolite to provide a constant impuritylevel to the gas flow stream for purposes of calibration would not beaffected by contact with the gas flow stream if the gas flow streamcontained impurity in the same approximate concentration range.

FIG. 5 is a schematic representation of a detector system 500 accordingto another embodiment of the present invention, similar to the detectorsystem of FIG. 4, and wherein all parts and elements corresponding tothe system previously shown and described with reference to FIG. 4 arecorrespondingly numbered, but by addition of 100 to the referencenumeral of the corresponding reference number of the same or similarpart or element in FIG. 4. This embodiment provides calibration of thesensor baseline and calibration of the detector sensitivity at multipleknown impurity concentration points, provided that a mass flowcontroller or other flow control device provides controllable andmeasurable gas stream flow rate. The ability to calibrate at multipleimpurity concentrations is desirable because the response curves of manyhighly sensitive sensor elements are not linear. In addition, thein-line detector may then be accurately calibrated in more than oneimpurity concentration regime to suit changing conditions.

The detector system in FIG. 5 operates similarly to that of FIG. 4, withthe key difference that instead of providing an impurity source bed thatimparts one known impurity concentration to the gas flow stream, thedetector system of FIG. 5 includes an impurity source such as one ormore permeation tube(s) 550 that, at a constant temperature, providesimpurity at a constant rate. Thus the impurity concentration in the gasflow stream may be controlled by varying the gas stream flow rate.

Surface acoustic wave (SAW) devices may be employed in the detectorsystems shown in FIGS. 4 and 5. SAW devices are usefully employed aschemical sensors in the broad practice of the present invention, due totheir ability to respond to changes caused by the binding of atoms andmolecules to the SAW device surfaces which lie in the path of thetransmitted acoustic wave. The response of the SAW device can takeeither the form of a surface wave amplitude attenuation, or, morecommonly, the form of a change in the wave velocity. These changes, inturn, result from the formation of a thick surface layer of boundmaterial (in the case of SAW devices used in the present invention, alayer of the impurity species bound to the affinity substrate), whichdiffers from the SAW substrate lacking such bound impurity species, interms of elasticity, mass density, viscosity, and/or conductivity.

SAW devices can be applied as gas sensors, as well as for sensing ofliquid deposited on the SAW affinity surface. For gas sensingoperations, a Rayleigh mode acoustic wave normally is employed; forliquid phase sensing, various acoustic wave modalities may be employed,including the Rayleigh mode, horizontal shear plate mode, or flat platemode acoustic waves. In the broad practice of the present invention, forgas impurity sensing, Rayleigh mode SAW gas sensors are preferablyemployed. Rayleigh mode waves yield the highest mass responsesensitivity and are the operating mode of choice for gas sensingapplications where wave attenuation is not a problem.

The high sensitivity of SAW devices in gas impurity sensing applicationsis well known and established in the art, and various reports have beenpublished demonstrating the utility of SAW devices in specificimpurity-containing gas systems. For example, Vetelino, J. F., et al,IEEE Trans. Ultrason., Ferroelec. Freq. Control, UFFC-34(2), 156-161(1987) describes the use of a SAW device having a surface coating of WO₃as employed for sensing of hydrogen sulfide, with sensitivity toconcentrations of hydrogen sulfide of less than 10 ppm. Venema, A., etal, IEEE Trans. Ultrason., Ferroelec. Freq. Control, UFFC-34(2), 148-155(1987) reports the use of a coating of copper phthalocyanine, an organicsemiconductor, on a SAW device to sense nitrogen dioxide (NO₂) with athreshold detection sensitivity of 500 Vui. Both of these reportedapplications involve the use of dual SAW device configurations tocompensate for nonspecific effects.

With reference to the specific impurity species-binding coating whichmay be employed in a SAW device sensor element in the detector systemsof FIGS. 4 or 5, an acceptable coating must meet the following criteria:

(1) the coating must reversibly bind the impurity species in a suitableconcentration range of interest (e.g., water in a 10-100 ppb range);

(2) the coating must bind the impurity in the presence of the reactivegases of interest, with a minimum of interference from the process gasstream;

(3) the coating must be non-contaminating with respect to the gasstreams of interest;

(4) the coating must be stable over an extended period of time; and

(5) the coating must be easily and reproducibly applied to the sensingsurface of the SAW device.

Even when considering a single impurity species such as water, it isapparent that no universal hygroscopic coating exists for all possiblegas streams of interest which may contain such (water) impurity. In thecase of water as an impurity, it is expected that inert gases andhydride gases of Group IV--VI elements of the Periodic Table will beable to use the same moisture-affinity coating. The impurity-affinitycoating employed for sensing of water impurity in hydrogen halide gases,however, will be different.

In the case of inert and hydride reactive gases (including those ofGroup IV--VI elements), a poly(vinylamine) coating may potentiallyusefully be employed in the broad practice of the present invention,provided that such coating meets the criteria (1)-(5) stated above.Amines are know to reversibly bind water, and, in fact, an amine coatingis employed in the aforementioned DuPont 5700 Moisture Analyzer which,as noted, has a sensitivity in the 10 ppb range, and has been employedsuccessfully in inert gas as well as arsine moisture-sensing service.

The molecular weight of the poly(vinylamine) is desirably high enough sothat the polymer will have no appreciable vapor pressure which wouldotherwise contaminate the gas stream being monitored for impurityconcentration. The poly(vinylamine) polymer is non-degradable in hydridegas streams, so that such polymer is non-contaminating in the gasstreams being monitored.

In addition, poly(vinylamine)s are soluble in polar solvents, so thatthey are readily applied to SAW device surfaces, e.g., by spin-coatingof polar solvent solutions of such polymers. Suitable poly(vinylamine)sare commercially available from Polysciences, Inc. (Warrington, Pa.) andmay be usefully employed in as-purchased form, for dissolution intopolar solvents and application to the sensor surface by suitableapplication techniques such as spin coating.

When moisture is the critical impurity to be sensed in hydrogen halidegas streams, a suitable coating material for the SAW device affinitysurface comprises poly(vinylsulfonic acid). This material is the acidequivalent to the poly(vinylamine) coating discussed above, and isusefully employable for affinity surface coatings on SAW devices inhydride gas service. Poly(vinylsulfonic acid) is commercially availablefrom Polysciences, Inc. and may be employed in as-furnished form bydissolution into a suitable solvent, and application, e.g., by spincoating, to the sensing surface of the SAW device.

The aforementioned illustrative polymeric materials employed to coat thesensor surface of the SAW device may be spun coated by aqueous solutionsthereof.

After the SAW device is coated with the impurity-affinity layer or film,the detection capability thereof may be tested using calibratedstandards such as the calibration device 600 shown and described withreference to FIG. 6 above. In the course of such calibration testing,the response of the coating is measured, and the coating is modified ifnecessary to achieve the desired highly sensitive, stable, andreproducible sensor layer required in the detection system of thepresent invention.

FIG. 6 is a schematic representation of an impurity standard calibrationdevice 600 as may be used in the practice of the present invention, forexample as impurity source 550 in FIG. 5. As shown in FIG. 6, thecalibration device 600 comprises a container 602 having disposed thereinan impurity constituent liquid 606. Above this impurity liquid is avapor space 608 in vapor flow communication with a permeation tube 604,which is formed of a permeable membrane material which allows theout-diffusion of the impurity species into the gas surrounding thepermeation tube. The permeation tube may be formed of a suitablepolymeric material having known and controllable permeabilitycharacteristics, e.g., polytetrafluoroethylene, or other suitablepolymeric or, alternatively, nonpolymeric material.

Consistent with the criticality of water as an impurity species insemiconductor manufacturing operations, the calibration device 600 shownin FIG. 6 may suitably contain as the liquid 606 a quantity of water.With the homogeneous structure of the permeation tube 604, the diffusionof permeant (water) at constant temperature will also be constant. Thewater vapor diffusing out of the permeation tube, through the surfacealong its length, then may be passed into a carrier gas, e.g., apreviously purified gas stream (with respect to impurity therein), toprovide a constant, known concentration of impurity for calibration ofthe SAW device or other sensor element in the practice of the presentinvention. The calibration device 600 shown in FIG. 6 affords a simple,and highly reducible, means for supplying known impurity levels atsub-ppm concentrations.

The permeation tube 604 of the calibration device may itself becalibrated either gravimetrically or by measuring the water level of theliquid 606 in container 602, using a commercially available tracemoisture analyzer, such as for example a DuPont 5700 Moisture Analyzer,available from E. I. DuPont de Nemours and Company (Wilmington, Del.),which is capable of detecting moisture levels as low as 10 ppb.

FIG. 7 shows output signal versus time for a hygrometer sensor exposedto high and low concentrations of moisture in an inert gas stream. Thedata of the front end were collected using a 12-bit a/d converter. Datacollected were plotted as bits with no linearization. The hygrometer wasexposed to 1.6 ppm moisture and 0.3 ppm moisture in a 2 liter per minuteinert gas stream. The bypass valve was cycled to provide 30 secondssample gas and 10 minutes zero gas. A change in the signal of about 20bits was observed upon 30 second exposure to 1.6 ppm moisture. Upon 30second exposure to 300 ppb moisture, about 4 bits of change wereobserved. For this sensor and contaminant, the limit of this techniquemay be about 300 ppb. There were only small changes in the signal belowthis level.

The switch between 1.6 and 0.3 ppm took place at about 90 minutes. Thebreak in data was very clean. There was no blurring of data between thetwo concentrations.

The data for two cycles are shown in FIG. 8 at the 1.6 ppm moisturelevel. This close-up of the data shows repeatable structure to thechanges in signal. At this time, the source and significance of thesecondary structure is not known.

Under the conditions of the test, the sensor had recovered to thebaseline noise within a range of 5 to 6 minutes. This recovery time isto be compared with a recovery time of several hours to recover fromstatic equilibrium with a gas stream containing ppm levels of moisture.

As mentioned before, the change in signal can be extrapolated to theequilibrium concentration. A calibration curve was made over the rangeof 300 to 3000 ppb. The curve is shown in FIG. 9, which shows signalchange (bits) versus ppb moisture, for 30 second exposures to the samplegas stream. The data appear to be roughly linear. However, a slightlybetter fit is obtained with a second order polynomial equation. Thequadratic term is quite small.

Calibrations may be carried out up to the 20 ppm range in thedifferential mode, for this sensor/contaminant system. These experimentsdemonstrate that the differential mode offers a fast response from thein-line monitor in high moisture conditions, without sacrificing thesensitivity capabilities of the detector. Instead of the multi-hourtimes that are required for recovery when operating in the normal mode,when operating in the differential mode, recovery times are in the 5-10minute range. This time can be reduced further without significantdifficulties.

The crossover between operation in the normal mode and in thedifferential mode may be manual or automatic. In most cases, the in-linemonitor is in normal mode for most of its operations. A centralprocessing unit (CPU), which may as previously described comprise anysuitable calculational means, collects the signal and calculates anaverage value and standard deviation σ, a measure of the noise in thesignal. If the CPU detects a change in signal greater than apredetermined amount, for example 5σ, the CPU resets the in-line monitorto operation in the differential mode. In the differential mode, thein-line monitor takes brief snapshots of the moisture level. Acalibration curve designed for the differential mode is used. When thehigh moisture level has dropped off, the CPU switches the in-linemonitor back to operation in the normal mode, where greater low levelmoisture sensitivity is found.

While the preceding description has been directed primarily to in-linemonitoring systems and methods utilizing sensor media of a reversiblysorptive character, i.e. physical sorbents or other reversible impurityspecies binding media, it will be recognized that the utility of theinvention is not thus limited. The invention also contemplates the useof chemisorbent sensor media, e.g., chemical getters, in which theimpurity species binding is irreversible, by virtue of chemical reactionor very high binding affinity constants. In such instances, theinvention may utilize the sensor in a differential mode of sensing ofgas containing impurity species, to prolong the useful life of thesensor medium beyond what would otherwise be possible in the use of suchaffinity material.

For example, the sensor may comprise a chemical getter material such asa metal or metal alloy, e.g., elemental barium, which binds reactivelyand irreversibly to gas impurity species such as oxygen, nitrogen,hydrogen, etc:

Various other features, aspects, and embodiments of the invention aredescribed in my prior co-pending U.S. application Ser. No. 07/930,184filed Aug. 17, 1992, now U.S. Pat. No. 5,825,705, which was acontinuation-in-part of my prior co-pending U.S. patent application Ser.No. 07/628,490, filed Dec. 14, 1990, and issued Aug. 18, 1992 as U.S.Pat. No. 5,138,869, the disclosures of which hereby are incorporatedherein by reference in their entirety, to the extent not otherwisepresent herein.

Although the invention has been described with respect to particularfeatures, aspects, and embodiments thereof, it will be apparent thatnumerous variations, modifications, and other embodiments are possiblewithin the broad scope of the present invention, and accordingly allvariations, modifications, and embodiments are to be regarded as beingwithin the spirit and scope of the invention.

What is claimed is:
 1. An in-line monitoring system for determining thecalibrated concentration of an impurity species in a flowing gas stream,in a low concentration range below a predetermined concentration value,and in a high concentration range above said predetermined concentrationvalue, said system comprising:(a) means defining a flow path for aflowing gas stream; (b) a gas purifier through which gas deriving fromthe flowing gas stream is flowable to remove impurity species therefrom;(c) means for sensing the impurity species concentration of gas derivingfrom the flowing gas stream; (d) means for selectively flowing gas fromthe flow path through the gas purifier to yield purified gas depleted insaid impurity species; (e) means for flowing purified gas from said gaspurifier to said sensing means; (f) means for selectively flowing gasfrom the flow path to the sensing means without passage of the gasthrough the gas purifier; (g) means operatively coupled to said sensingmeans for determining a calibrated equilibrium value of impurity speciesconcentration in the flowing gas stream from impurity speciesconcentration in unpurified gas sensed by said sensing means incalibrative relationship to impurity species concentration of purifiedgas sensed by said sensing means; (h) flow control means operativelycoupled with gas flow means (f) and (g), and arranged to selectively,alternatively and repetitively flow gas deriving from the flowing gasstream through gas flowing means (f) to the gas purifier for a firstselected period of time t₁, and subsequently through gas flowing means(g) for a second selected period of time t₂, in a repeating cycle,wherein in the low concentration range below said predeterminedconcentration value, t₂ >t₁, and wherein in the high concentration rangeabove said predetermined concentration value, t₁ >t₂.
 2. An in-linemonitoring system according to claim 1, wherein in the low concentrationrange below said predetermined concentration value, 0.50<t₂ /(t₁ +t₂)≦0.99, and wherein in the high concentration range above saidpredetermined concentration value, 0.5<t₁ /(t₁ +t₂)≦0.99.
 3. An in-linemonitoring system according to claim 1, wherein the sensor means (c)comprise physical sorption sensor means, and wherein in the highconcentration range above said pre-determined concentration value, thesecond selected period of time t₂ is selected to allow dynamic contactequilibrium to occur but not static equilibrium.
 4. An in-linemonitoring system according to claim 1, wherein the sensing means (c)comprise a physical sorption medium having sorptive affinity for theimpurity species.
 5. An in-line monitoring system according to claim 1,wherein the gas purifier comprises a chemisorbent material which ischemically reactive with the impurity species to remove same from thegas deriving from the flowing gas stream.
 6. An in-line monitoringsystem according to claim 1, wherein the sensing means (c) comprise ahygrometric sensor.
 7. An in-line monitoring system according to claim1, wherein the means (g) for determining a calibrated equilibrium valueof impurity species concentration in the flowing gas stream fromimpurity species concentration in unpurified gas sensed by said sensingmeans in calibrated relationship to impurity species concentration ofpurified gas sensed by said sensing means, comprises a digital computer.8. An in-line monitoring system according to claim 1, wherein the means(g) for determining a calibrated equilibrium value of impurity speciesconcentration in the flowing gas stream from impurity speciesconcentration in unpurified gas sensed by said sensing means incalibrated relationship to impurity species concentration of purifiedgas sensed by said sensing means, comprises a microprocessor.
 9. Anin-line monitoring system according to claim 1, wherein the flow controlmeans (h) comprise an automatic valve operatively coupled with gasflowing means (f) and (g).
 10. A method for determining the calibratedconcentration of an impurity species in a flowing gas stream, in a lowconcentration range below a predetermined concentration value, and in ahigh concentration range above said predetermined concentration value,said method comprising:providing an in-line monitoring systemcomprising: (a) means defining a flow path for a flowing gas stream; (b)a gas purifier through which gas deriving from the flowing gas stream isflowable to remove impurity species therefrom; (c) means for sensing theimpurity species concentration of gas deriving from the flowing gasstream; (d) means for selectively flowing gas from the flow path throughthe gas purifier to yield purified gas depleted in said impurityspecies; (e) means for flowing purified gas from said gas purifier tosaid sensing means; (f) means for selectively flowing gas from the flowpath to the sensing means without passage of the gas through the gaspurifier; (g) means operatively coupled to said sensing means fordetermining a calibrated equilibrium value of impurity speciesconcentration in the flowing gas stream from impurity speciesconcentration in unpurified gas sensed by said sensing means incalibrative relationship to impurity species concentration of purifiedgas sensed by said sensing means; (h) flow control means operativelycoupled with gas flowing means (f) and (g), and arranged to selectively,alternatively and repetitively flow gas deriving from the flowing gasstream through gas flowing means (f) to the gas purifier for a firstselected period of time t₁, and subsequently through gas flowing means(g) for a second selected period of time t₂, in a repeating cycle,wherein in the low concentration range below said predeterminedconcentration value, t₂ >t₁, and wherein in the high concentration rangeabove said predetermined concentration value, t₁ >t₂ ; flowing gas fromthe flowing gas stream selectively from the flow path through the gaspurifier to yield purified gas depleted in said impurity species,alternatingly with flowing gas from the flow path through the gasflowing means (f), and passing the gas to said sensing means; andselectively operating said flow control means (h) to flow gas throughmeans (f) to the gas purifier for a first selected period of time t₁,and subsequently through gas flowing means (g) for a second period oftime t₂, in a repeating cycle, wherein in the low concentration rangebelow said predetermined concentration value, t₂ >t₁, and wherein in thehigh concentration range above said predetermined concentration value,t₁ >t₂.
 11. A method according to claim 10, wherein in the lowconcentration range below said predetermined concentration value,0.50<t₂ /(t₁ +t₂)≦0.99.
 12. A method according to claim 10, wherein thesensing means comprises a physical sorption sensor, and wherein in thehigh concentration above said predetermined concentration value, thesecond selected period of time t₂ is selected to allow dynamic contactequilibrium to occur but not static equilibrium.
 13. A method accordingto claim 10, wherein the sensing means (c) comprise a physical sorptionmedium having sorptive affinity for the impurity species.
 14. A methodaccording to claim 10, wherein the gas purifier comprises a chemisorbentmaterial which is chemically reactive with the impurity species toremove same from the gas deriving from the flowing gas stream.
 15. Amethod according to claim 10, wherein the sensing means (c) comprise ahygrometric sensor.
 16. A method according to claim 10, wherein themeans (g) for determining a calibrated equilibrium value of impurityspecies concentration in the flowing gas stream from impurity speciesconcentration in unpurified gas sensed by said sensing means incalibrated relationship to impurity species concentration of purifiedgas sensed by said sensing means, comprises a digital computer.
 17. Amethod according to claim 10, wherein the means (g) for determining acalibrated equilibrium value of impurity species concentration in theflowing gas stream from impurity species concentration in unpurified gassensed by said sensing means in calibrated relationship to impurityspecies concentration of purified gas sensed by said sensing means,comprises a microprocessor.
 18. A method according to claim 10, whereinthe flow control means (h) comprise an automatic valve operativelycoupled with gas flowing means (f) and (g).
 19. A method of determininga calibrated concentration of an impurity species in a flowing gasstream, in a low concentration range below a predetermined concentrationvalue, and in a high concentration range above said predeterminedconcentration value, said method comprising:sensing the impurity speciesconcentration of gas derived from the gas flow stream in a sequentialand repetitive sensing operation wherein in a first sensing mode the gasis purified of impurity species prior to sensing thereof and wherein ina second sensing mode the gas is unpurified, and wherein in the firstsensing mode and second sensing mode, the sensing mode cycle times arevaried in accordance with the impurity species concentration, so that insaid low concentration range below said predetermined concentrationvalue, the second sensing mode cycle time is greater than the firstsensing mode cycle time, and wherein in said high concentration rangeabove said predetermined concentration value, said first sensing modecycle time is greater than the second sensing mode cycle time; anddetermining a calibrated equilibrium value of impurity speciesconcentration in the flowing gas stream from sensing concentrations inthe first and second sensing modes.
 20. An in-line monitoring system fordetermining a calibrated concentration of an impurity species in aflowing gas stream, in a low concentration range below a predeterminedconcentration value, and in a high concentration range above saidpredetermined concentration value, said system comprising:a purifier forremoving impurity species from gas derived from the flowing gas stream;a means for sensing the impurity species concentration of gas derivedfrom the gas flow stream in a sequential and repetitive sensingoperation wherein in a first sensing mode the gas is purified ofimpurity species by passage thereof through said purifier prior tosensing thereof and wherein in a second sensing mode the gas inunpurified, and wherein in the first sensing mode and second sensingmode, the sensing mode cycle times are varied in accordance with theimpurity species concentration, so that in said low concentration rangebelow said predetermined concentration value, the second sensing modecycle time is greater than the first sensing mode cycle time, andwherein in said high concentration range above said predeterminedconcentration value, said first sensing mode cycle time is greater thanthe second sensing mode cycle time, and a means for determining acalibrated equilibrium value of impurity species concentration in theflowing gas stream from sensing concentrations in the first and secondsensing modes.
 21. A method of determining a calibrated concentration ofimpurity species in a flowing gas stream, wherein gas derived from thegas flow stream is alternatingly sensed as to its impurity speciesconcentration in purified and unpurified conditions, and said sensingcomprises contacting of gas with a sensor including a binding mediumhaving irreversible binding affinity for the impurity species, saidmethod comprising sensing impurity species concentration of unpurifiedgas by said sensor in a differential mode, and controlling (1) sensingtime of the sensor in exposure to said unpurified gas, relative to (2)sensing time of the sensor in exposure to purified gas, in response toimpurity species concentration of unpurified gas sensed by said sensor,wherein sensing time of the sensor in exposure to said unpurified gas isless than 50% of the operating time of the sensor, whereby the sensorservice life is extended relative to continuous sensing of unpurifiedgas.
 22. An in-line monitoring system for determining a calibratedconcentration of impurity species in a flowing gas stream, comprising:asensor including a binding medium having irreversible binding affinityfor the impurity species, for sensing impurity species concentration ina gas; means for purifying gas derived from said flowing gas stream toyield purified gas; means for flowing unpurified gas derived from saidflowing gas stream to said sensor and for flowing purified gas from saidgas purifying means to said sensor, such that said unpurified gas issensed by said sensor in a differential mode wherein sensing time of thesensor in exposure to said unpurified gas is less than 50% of theoperating time of the sensor, whereby the sensor service life isextended relative to continuous sensing of unpurified gas; means fordetermining a calibrated concentration of impurity species in saidflowing gas stream from impurity species concentrations of said purifiedand unpurified gas sensed by said sensor; and means for controlling (1)sensing time of the sensor in exposure to said unpurified gas, relativeto (2) sensing time of the sensor in exposure to purified gas, inresponse to impurity species concentration of unpurified gas sensed bysaid sensor.